Many objects are so fragile that contacting them at all subjects them to damaging stress. Some fragile surfaces, however, can withstand a light force and therefore can be subjected to direct measurement in which contact occurs.
This ability to contact fragile objects is of particular importance in the manufacture of semi-conductor packages which are applied to the surface of a printed circuit board. When a plurality of leads extend from a single device, it often times is necessary to determine if all of the leads are appropriately arranged for contact on the printed circuit board. Particularly when multiple leads extend in the same direction, so that the device is mounted on the surface of the printed circuit board and is suspended above the board by the leads, it is necessary that these leads all make contact with the surface of the PC board. It is particularly important to have complete contact with all of the leads since each lead is essential for the total functionality of the device.
Nevertheless, there is no conventional measurement means to determine whether or not all of the leads of a surface mounted device (SMD) are in the same plane, so that appropriate and effect contact with the surface of the printed circuit board can be achieved. Linear probes and micrometers have not been successful in achieving the degree of accuracy needed for error free manufacturing. Also, use of these inadequate methods as described above can often times cause more of the individual leads to be non-aligned and therefore causes more problems than it solves.
One device which has been suggested is shown in U.S. Pat. No. 4,774,768, in which leads contact a non conductive post to overcome a spring or bias. In this patent a non conductive spacer is used to verify that all of the leads break contact with the lead within the thickness of that spacer. However, if the spacer is too thin, false reading are made and if it is too thick, the data is meaningless. In any event, even with an approximately sized spacer, coplanarity is really not being measured.
Presently, non-contacting calibration methods are employed in which lasers are aligned to measure an absolute distance. However, these designs require very careful alignment and are extremely expensive as well. A totally automated system using a laser would be prohibitively expensive. It would be a great advantage to the art if a device could be provided which would allow contacting the various leads of a SMD prior to mounting it on a printed circuit board to determine that all of the leads are within the required degree of coplanarity. In this manner, the mounting of the SMD would be highly reliable and effective during automated assembly of the completed device.
With the foregoing in mind, it is an object of the present invention to provide a method for accurately determining the coplanarity of the individual leads in a SMD.
Another object of the present invention is to provide apparatus which is suitable for accurately measuring deviations of various points in a plane from coplanarity of that plane.
Still another object of the present invention is to provide a device which can automatically display the location of individual leads on an SMD which are outside of an acceptable limit for coplanarity.
Yet another object of the present invention is to provide a signal device for use for with SMD and other sensitive or fragile articles of manufacture.